All nervous systems employ a large umber of amines, amino acids and neuropeptides as neurotransmitters and neuromodulators. Despite years of work on individual brain signaling systems, the organizational principles underlying neuromodulation remain incompletely understood. The proposed work will employ the crustacean stomatogastric nervous system to address a series of general questions about neuromodulation of brain circuits. Mass spectrometry will be used to determine the identity of neuropeptides in specific sensory and modulatory neurons. The function of the specific neuropeptides found in identified neurons will be examined electrophysiologically. In cases in which multiple members of a peptide family are found in the same identified neurons, the potency and action of the different family members will be compared. To determine how a single neuron organizes receptors for 12 or 15 different neurotransmitters or modulators, neurons will be filled with fluorescent dye, and agonists applied to various regions of the neuron to localize its receptors. Further experiments will characterize the voltage-dependence and pharmacological profile of the evoked responses. The effects of neuromodulators on circuit reorganization will be studied with quantitative methods that determine how different frequency rhythms interact. Additionally, the effects of neuromodulators on signal propagation will be investigated by determining the extent to which externally imposed signals (sine waves of various frequencies) influence the activity of circuit neurons. Together these data will contribute to basic understandings of how different neuromodulatory systems can modify networks to produce behaviorally relevant changes in function.